Update Knative version to 0.6

Problem to solve

In order to consume the latest Knative updates, we need to bump our reference Knative version to 0.6

Intended users

Operators, developers

Further details

Proposal

Update Knative application to 0.6
app/models/clusters/applications/knative.rb
spec/models/clusters/applications/knative_spec.rb

Permissions and Security

Documentation

Testing

What does success look like, and how can we measure that?

Links / references

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28798